As a method for measurement of frame loss of a path set on a network, ITU-T Recommendation Y.1731 discloses in section 8.1 the measurement of frame loss of a service path set on the Ethernet® (ETH-LM: Ethernet® Loss Measurement). In the disclosed ETH-LM, the number of frames transmitted at a transmission side MEP (maintenance entity group end point) is counted by a transmission counter. Further, an OAM (operation, administration, and maintenance) frame carrying the value of the transmission counter is added to the data stream at a period of about 100 msec. The reception side MEP counts the number of received frames by a reception counter and compares the difference of the reception counter value at the time of reception of the OAM frame from the time of reception of the previous OAM frame and the difference of the transmission counter value contained in the OAM frame from that previous value so as to calculate the frame loss.
This literature discloses the two techniques of dual ended ETH-LM and single ended ETH-LM. The dual ended ETM-LM uses a CCM (continuity check message) frame as an OAM frame and sends the CCM frames from both MEPs constantly to the other MEPs by a predetermined period and measures the frame loss in both directions at both MEPs. The single ended ETH-LM uses an LMM (loss measurement message) frame and a reply to the same, that is, LMR (loss measurement reply) frame, as OAM frames, and send an LMM frame from one MEP to the other MEP periodically and receives an LMR frame in reply, and measures the frame loss of the path in both directions based on information contained in the LMR frame.
In dual ended ETH-LM, a CCM frame includes the next counter values, that is, the value of the transmission counter at the time of transmission of the CCM frame, that is, TxFCf, the value of the reception counter at the time of reception of the finally received CCM frame, that is, RxFCb, and the value of TxFCf which was carried in the finally received CCM frame, that is, TxFCb. The MEP receiving this CCM frame uses the difference of these values from the previous values and the difference of the value of the reception counter at the time of reception of the CCM frame from the value at the time of the previous reception to calculate the frame loss of the path in both directions so as to thereby calculate the frame losses in both directions at both MEPs.
In single ended ETH-LM, an LMM frame carries the value of the transmission counter at the time of transmission of the LMM frame, that is, TxFCf. Further, the LMR frame in reply to the LMM frame, carries the TxFCf copied from the LMM frame, the value of the reception counter at the time of reception of the LMM frame, that is, RxFCf, and the value of the transmission counter at the time of transmission of the LMR frame, that is, TxFCb. The MEP receiving this LMR frame calculates the frame loss of the path in both directions from the difference of these values from the previous values and the difference of the value of the reception counter at the time of reception of the LMR frame from the value of the previous time of reception.
The above two techniques are common in the fact of making an OAM frame (CCM, LMM, or LMR frame) carry the value of the transmission counter at the time of transmission of that OAM frame itself.
Usually, the generation of the OAM frame and the transmission of the OAM frame are performed by separate functional units operating in parallel. For that reason, after reading out the value of the transmission counter for generation of an OAM frame, there is the possibility of a data frame being transmitted before the OAM frame which carries the read out value is actually transmitted. In other words, there is a possibility of the value of the transmission counter at the time the OAM frame is generated and the value of the transmission counter at the time the OAM frame is actually transmitted not matching. Data frames do not flow at constant periods, so that deviation will not be fixed. Therefore, the difference of the transmission counter value contained in the OAM frame from the previous value might not match the number of data frames actually sent between two OAM frames.
The above Recommendation, page 31, also describes the following as NOTE 1:
“The level of accuracy in the loss measurements is dependent on how frames with ETH-LM information are added to the data stream after the counter values are copied in the ETH-LM information. For example, if additional data frames get transmitted and/or received between the time of reading the counter values and adding the frame with ETH-LM information to the data stream, the counter values copied in ETH-LM information become inaccurate.”
Further, there is the following description as a countermeasure:
“However, a hardware-based implementation which is able to add frames with ETH-LM information to the data stream immediately after reading the counter values, provides enhanced accuracy.”
However, it is impossible or remarkably difficult to reduce the processing time, from generation of an OAM frame to its transmission, to zero even if realized by hardware, which is liable to invite greater complexity and larger size of the circuit and a drop in the processing speed.
Non-Patent Literature 1: ITU-T Recommendation Y.1731 (8.1)